The present invention relates to wireless communications and particularly to Radio Wave Antennas.
Efficiency of a transmitting antenna may be defined as the ratio between the power the antenna radiates and the power put into the antenna by a coupled transmitter. Usually, high efficiency is desirable in an antenna.
The physical size of an antenna, normalized to its operating wavelength, usually refers in the art as the “electrical size” of the antenna, so a “small antenna” usually means an Electrically Small Antenna (ESA). Typically, small antennas are desirable, particularly in mobile and portable devices, since enable to implement, carry and operate compact and user friendly devices.
Ideally, a small and efficient antenna should suit most wireless devices, however, a well-known rule trades off between these two aspects, limiting the miniaturization of an antenna, for a given efficiency. This rule also indicates that at least one of the antenna dimensions should be not less than λ/4, where λ (lambda) is the transmission or reception wavelength, to achieve efficient radiation. For example, a λ/4 monopole structure, with a “whip” or “rod” about λ/4 long, electromagnetically coupled to a ground plane. For a 406 MHz radio λ/4 means 185 mm.
While smaller than λ/4 monopole antennas can be configured, this usually degrades the antenna efficiency. Thus, an efficient antenna for low frequency is not easily achieved in small dimensions.
Over the years, more complex shapes of antennas, many of them three dimensional, were been studied. Some fundamental works were been published by Wheeler [H. A. Wheeler, “Fundamental Limits of Small Antennas,” Proceedings of The I.R.E. (IEEE), December 1947, pg. 1479-1484], Chu (Chu, L. J, “Physical Limitation of Omni-Directional Antennas”, Journal of Applied Physics, Vol. 19, p. 1163-1175, 12/1948) and others. Based on these works, theoretical arguments predict that the minimal size for practical antennas will require a volume of half a sphere with radius r, where kr=0.3 (k=2π/λ). For example, at 406 MHz this means r˜40 mm. In part of the literature the radius of this sphere is named a instead of r, so ka=0.3 is considered the minimum figure for an efficient ESA.
Yet, electrically small antennas not only suffer from some degradation in efficiency but also usually obtain a narrow bandwidth, which makes the ESA vulnerable to de-tuning due to proximity to metallic bodies, heavy snow, etc.
Not surprisingly, the present art covers several methods to configure antennas, composed of relatively movable parts or of flexible material so that could be folded or collapsed to occupy less space when not in use. Such methods are particularly popular in the military and satellite communications, for obvious reasons.
U.S. Pat. No. 4,115,784 to Schwerdtfeger, et al. discloses a deployable ground plane antenna for use aboard a satellite or the like, with the antenna and erection mechanism being compactly stowable within the confines of a launch vehicle. After ejection of the satellite from the launch vehicle, the ground plane antenna self-deploys on removal of a single cable restraint.
U.S. Pat. No. 5,909,197 to Heinemann et al. discloses a Deployable helical antenna, comprised of a top and a bottom plate, and a deployable structure fitted between the plates which can forcibly separate the plates and extend a helical antenna placed between the plates.
Yet the structures disclosed by Schwerdtfeger and Heinemann are quite big and complex, perhaps suitable to satellites but less practical for small and portable devices.
In the recent years, portable communication devices for personal use were introduced to the market, operating at relatively low frequencies, such as VHF and low UHF. One prominent case is about Personal Locator Beacons (PLBs) for Search and Rescue (SAR) of people in distress. Some of these PLBs operate at 121.5 MHz, i.e. λ=2470 mm, others operate at 406 MHz, i.e. λ=740 mm Concerning these wavelengths, a λ/4 antenna is hardly compact enough to be part of a personal device carried or worn by a user, particularly when the user is required to use his/her hands while swimming or climbing, and so on. For such uses, deployable or collapsible antennas may provide a fair solution also for small and portable communication devices, since the antenna could be most of the time stowed in a low profile, and deployed, when needed to achieve full radiation performance.
U.S. Pat. No. 5,559,760 to Schneider (Breitling) discloses a wristwatch comprising a high-frequency transmitter and an extensible antenna in the form of two wires wound up in two different housings of the watch before use; the antenna being unfurled by pulling on plugs fastened to each end of the antennas. The dipole antenna of this device is configured that once been extended, does not flex but remains straight. Yet, Schneider's dipole antenna is quite long and not so friendly to use.
U.S. Pat. No. 7,586,463 to Katz discloses Extendable helical antenna for personal communication device. According to Katz, a helical antenna is placed over a ground plane, packaged in a case with a rigid cover, the helical antenna made of an elastic conductive spring configured to change its height along its axis, pressed down between said case and said cover or extended to a higher height. Still, the small helix antenna disclosed by Katz, known to operate in the normal mode, poorly radiates to the zenith, so is limited in communicating with high elevation satellites.
U.S. Pat. No. 7,038,634 to Bisig discloses a loop antenna embedded in a wristband portion of a watch. This invention provides a compact solution to an antenna embedded in a wrist watch, however it is typically applied to receive FM radio broadcast, and is not efficient enough to be used in a transmitter, and neither obtains a radiation pattern with good performance at high elevation angles, so is not very practical for communication with high elevation satellites.
A wrist worn transmitter is particularly useful for Search and Rescue of people in distress. Adventurers, travelers, boaters, sailors, pilots, and outdoorsmen run a constant risk that something will go wrong, and will subsequently find themselves in need of rescue services. Such people require a device that can call for rescue in emergency situations, whether at sea, on land, or in the air.
There are already such devices in the market, named Personal Locator Beacons (PLBs), made to broadcast distress signals detectable by satellites and relayed to terrestrial search and rescue centers. The most prominent satellite system for Search and Rescue is Cospas-Sarsat (C/S), operating worldwide since 1982 and been instrumental in saving about 40,000 people from then, by detecting and locating signals broadcast from PLBs or alike beacons/terminals (named EPIRBs and ELTs).
Emergency situations relevant to Search and Rescue arise by nature at unexpected moments, when the user might not be prepared, such as when falling overboard a vessel, in snow avalanche, or due to equipment failure during climbing. Then, it is mostly desirable that a PLB, detectable by satellites, be attached to the user's body on a permanent basis. In particular, a wrist worn PLB is mostly practical in such cases. Furthermore, a small and efficient and robust antenna is vital for such applications.
State of the art Search and Rescue satellite systems use independent methods to locate the beacon, not necessarily based on Global Navigation Satellite Systems (GNSSs) positioning services. For example, the beacon localization in the Cospas-Sarsat system segment named MEOSAR (Medium Earth Orbiting Search and Rescue) is based on the capability of a base station to monitor the beacon signal been relayed by three or more satellites. Measuring the relayed signal Time of Arrival (ToA) and Frequency of Arrival (FoA) provides information enabling resolving the beacon position. Since this MEOSAR employs Medium Earth Orbiting satellites to relay signals broadcast from terrestrial beacons, and since these satellites are deployed substantially symmetrically around the earth, it would be mostly desirable that beacons activated from the earth, will obtain a radiation pattern covering all the visible sky, i.e. any elevation angle above the horizon up to the zenith. In other words, the required antenna should be isotropic, or at least semi isotropic, covering the hemisphere above the horizon.
Since antennas have no active components, they do not amplify RF energy or increase the overall signal energy provided by a coupled transmitter; rather antennas shape the direction of the radiated energy into a specific pattern. Wired Dipole and Monopole antenna generate an Omni-directional (in azimuth) toroidal shaped wave pattern, i.e. reducing power at high elevation angles, measured from the horizon, vanishing towards the zenith and nadir.
The isotropic radiation pattern can prevent deterioration of communication quality caused by nulls. Thus, antennas with isotropic radiation pattern are very adaptable to portable communication products, especially handheld products and furthermore to satellite terminals, where signals are expected to arrive from or be transmitted to all directions in azimuth, but also to all elevation angles.
U.S. Pat. No. 7,948,446 to Barone discloses a XYZ isotropic radiator antenna characterized by three whip antennas connected with a housing and arranged 90 degrees perpendicular to each other. Yet, the three whip antennas turn to be quite long in VHF and UHF and could hardly be considered as an Electrically Small Antenna for a small portable communication device.
U.S. Pat. No. 8,264,418 to Huang discloses a planar antenna with an isotropic radiation pattern, including a substrate, a dipole antenna, a microstrip line set, and a channel selection module. The dipole antenna is disposed on a first surface of the substrate, and the microstrip line set and the channel selection module are disposed on a second surface of the substrate. However, this antenna is likely to have low efficiency, due to high self-loading in substrate and microstrip geometry.
U.S. Pat. No. 8,390,516 to Parsche discloses a Planar antenna having isotropic radiation pattern and based on an epicyclic structure. The antenna device includes an electrical conductor extending on a substrate and having at least one gap therein, and with an outer ring portion to define a radiating antenna element, and at least one inner ring portion to define a feed coupler and connected in series with the outer ring portion and extending within the outer ring portion. Yet this antenna obtains a 0.124 wavelength (lambda) diameter (without further loading), which still seems to be too large for a wrist worn device at VHF or UHF; also, this antenna radiates in Horizontal polarization, which might not match the typically Vertical polarization of the remote antenna with which it is desired to communicate; moreover, the dipole nature of Parsche's antenna, opposed to a monopole that uses a ground plane reference, might also be a disadvantage in the presence of nearby metallic bodies.
Present art methods have not yet provided satisfactory solutions for efficient isotropic radiating electrically small antennas, particularly in VHF and UHF bands.
It is then an object of the present invention to provide a portable communication device and antenna thereto, the antenna obtaining high efficiency and substantial isotropic radiation pattern.
It is another object of the present invention to provide a communication device and antenna well adopted to communicate with and be located by satellites viewed at any elevation above the horizon.
It is also an object of the present invention to provide a communication device and antenna, both compact and user friendly enough to be carried and operated by a person.
It is still an object of the present invention to provide a communication device and antenna compact and user friendly enough to be carried by a person engaged in physical activity, and operated in emergency situations.
It is yet another object of the present invention to provide a communication device and antenna suitable for a wrist-worn Personal Locator Beacon (PLB) served by satellites.
It is yet an object of the present invention to provide a communication device and antenna enabling a satellite base station to use as many satellites, in the most possible favorable geometry, to accurately determine the location of said communication device.
Other objects and advantages of the invention will become apparent as the description proceeds.